Blade of screw propellers



APril 14, 1931- F. MELCHEI ET AL. 1,800,313

BLADE OF. SCREW PROPELLERS Filed June 23, 1950 Patented Apr. 1 4 1931.

-UNITED STATES VPATENTJOFGFICE FRANZ MELCHER AND ERNST SCHNEIDIirR, OF VIENNA, AUSTRIA BLADE F scnnwrn'ornnnmts Application filed June 23, 1930, Serial No. 463,319, and in Austria May 28, 1929.

This invention relates to a new and improved design for the blades of' screw-pro-- vp pellers of every kind operating in any medium, and has for its object to achieve the 5 greatest possible efficiency and load capacity of the propeller in whichit is incorporated. The invention consists essentially in a particular plotting of the generating guide line of the twisted blade surface by means of which the holding capacity of the blades of the propeller is increased while cavitation at the boss is at the same time checked.

The generating guide line above and hereinafter mentioned is the intersecting line of a plane drawn through the propeller axis and the outer most blade point with the surface of the blade.

One known expedient forthe increasing of the holding capacity of propeller blades and for the recovery of the energy of fiow consists in the employment of nozzleor suction-pipe;shaped jacketing of the blade shells of turbines or of enclosed propellers.

In practise, however, this method has been abandoned, since the gain in thrust obtained by the use of the nozzle is .more than outweighed by skin-friction and tow-resistance.

A greatvariety of further expedients have been tried for the purpose of improving propellers, mostly relating to the pitch, outline,

1 radial'or peripheral curvature, or to the inclination of the propeller blades towards or away-from the direction of travel. The attempt hastalso been madeto increase the efficiency. of propellers by designing blades of unusual thinness, but these again have proved to be disadvantageous in consequence of comparatively slight draughtv and the development of cavitation at heavier loads. All these and similar attempts hitherto made with ithe object of improving upon the normal designs of propeller blades have led to no advance of general value, the

advantage gained in any particular direction I in any one case being. outweighed by dis-,

advantages in other dlrectio ns. I

The tests and calculations upon which the present invention'is based have pi oved that the ploting-and shaping of the generating I lines of the blade offers the possibility of improving upon even the, most perfect proeller of given characteristic. This conclusion'is derived from the followingbasic facts and considerations.

that a backward curve of the generating lines away from the direction of travel reduces the thrust of the propeller, since the convergence of the tail-water is thereby promoted, while the fluid pressure at the boss is increased and the danger of cavitation in this region reduced. This expedient therefore leads to a reduction in the holding capacity and thus also in the thrust of the propeller. The provision of a tubular suction nozzle enclosing the propeller would on the other hand (other factors being equal) increase the thrust, since" the convergence of the tailwater is thereby counteracted. On the other hand the forward curving of the guide line towards the direction of travel, have proved to result in an increased thrust, but at the same time in an increased liability to cavitation at the boss, since the whole blade in consequence-of its jet-like action counteracts the constriction of the tail-water and can generate nd centripetal falling-off of the pressure in the region of the boss.

Thepresent invention provides a special shaping of the generatingflines of the blade combining the advantages of the above-de scribed principles of design' while avoiding thedisadvantages of the same. This shaping ot the blades provided by "the present invention may be defined as follows. Thecgenerating line of the surface of the blade is inclined or curved in the region of the transition of the blade into the" boss of the'propeller, this'is to say for ap' proximately the first third of the length of 99 the blade from the bossto the tip, in the direction opposed to that of the thrust,'and in the last third of this length towards the direction of the thrust. It follows that in the remaining middle, third of this length 95 V the direction is r'nainlyaxial. This principle of designing'the blades enables all the advantages partially and singly obtained bf the above-described expedients to be attained simultaneously. The substantially Tests with a water propeller have proved 55 v J the case of other forms of construction; the

'radial direction is maintained, thus taln'ng account of the fact proved by experience that the propeller-blade the generating lines of which are of'axial direction possessesthe greatest strength and resistance to inherent vibration. The forwardly bent outer portion of the blade enables a comparatively greater quantity of water to be held than in the thrust.

Further the backwardly curved guide line in the inner third of the blade gives rise to a centripetal pressure component, thus eliminating cavitation at the boss.

1s obtuse.

An example of a screw-propeller de-,

signed in accordance with the present invention is shown in longitudinal section in the accompanying drawing. The. screwpropeller, which rotates about the axis 1, is provided with blades 2 which are so shaped that the generating line 3 or the locus 4 of the centres of gravity of the cross-sections of the blade is inclined at the base of the blade in adirection away from the direction of travelffi; towards the middle of the blade at 7 the generating line gradually as- 35 \sumes a direction at right angles to the axis of the propeller; and in' the outer third of its length the generating line gradually acquires an increasing inclinationor curvature in the direction of travel, so that the tip8 of the blade is located-furthest towards the front. In this example of a} form of construction shown in the drawing the line joining the base 5 of the blade with the tip 8 of the same is at right angles to the axis ofthe propeller, but the angle of inclinatlon a may equally'well be greater or less than 90F The essential feature, of every) possible embodiment of the invention is that the angle of inclination ,8 between the tan ant to the generating line at the base 5 o the blade and the axis of rotation of the propeller on the side towards the sternis an acute angle,

while the correspondin angleof the tangent at the tip of the blade The resulting shape of the generating lines is naturally with the concave sililtowards the front. The main direction 0 should be approximately at rightang'les to the axis of rotation of the propeller, although slight deviations' are peljnissible, so that the tip 8 of the blade may be somewhat in front of or behind the base-- 5- of the blade considered in an axial'dir'ection. -A Zembodying a blade tip in advance of the e mid-portion of the. generating lines,

\ blade base has the advantage over other types that the bending component which results from centrifugal force in the blade and acts counter to the thrust of the propeller reduces the total strain on the material of the propeller and enables the cross-, sections of the blades to be kept thinner than would otherwise be the case.

If the above described conditions be observed together with the known principlesand proportions regarded as ruling in respect to diameter, pitch, profile, superficial area, number of blades, etc., in the designing of propellers the possibility is given by the present invention, by the mere determining of the course and shape of the generating lines of the blade surface, to improve any and every type of propeller both as to-efiiciency and load capacity. The improvements. achieved in this manner are valid 'for the entire range of the permissible speeds.

'A further consequence of the eifective gains thus effected is that for a given thrust the superficial area of the blades can be reduced, which leads to a further increase in We claim 1; In' a screw-propeller having a boss, concave blades as viewed from the suction side and carried by the boss, saidblades being characterized by the essential feature.

that the generating line of the said blades is inclined in the first third of the radial length of .the said blades adjacent the boss of the said propeller in a direction away from the direction of travel of the inthe next following middle third of the said length in a direction coinciding with the direction of travel at right angles to the axis of rotation of the said propeller, and in the final outerthird of the said length in a direction towards the direction of travel.

2. In a screw-propeller having a "boss, concave blades as viewed from the suction side and carried by the boss; said blades bepropeller,

ing characterized by the essential feature.

that the generating line of said blade is inclined in the first third of: the radial length of said bladesadjacent the boss in the'direction of the thrust of the propeller, in the next following middle third of said length g in a direction coinciding with the normal direction of travel at rlght angles to the axis of rotation of the said propeller and b in the final outer third of said length in a direction towards the direction of travel.

signatures.

' In testimony whereof we have afiixed our FRANZ MELCHER, ERNST SCHNEIDER.

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